Externally activated seal system for wellhead

ABSTRACT

A method and apparatus for a seal assembly for a unitized wellhead system for land or platform applications utilizes a friction grip technology to create maintainable metal-to-metal seals with finely-controlled contact stresses, lock-down casing and tubing hangers, support test loads to minimize the size of landing shoulders required, and to rotationally lock casing hangers to provide simplified running procedures. The system can be used in combination with a friction grip clamping assembly to greatly streamline the wellhead design.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of and claims priority of co-pendingutility application Ser. No. 10/751,244 filed Dec. 31, 2003 entitled“Externally Activated Seal System for Wellhead.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is related to concentric casings and strings in wellheadswherein it is necessary to effect a seal between concentric members ofthe wellhead and is specifically directed to a seal system wherein thesealing members are activated via an external, non-invasive sealenergizing system.

2. Discussion of the Prior Art

In oil and gas wells, it is conventional to pass a number of concentrictubes or casings down the well. An outermost casing is fixed in theground, and the inner casings are each supported from the next outercasing by casing hangers which take the form of inter-engaging internalshoulders on the outer casing and external shoulders on the innercasing.

Typically, such casing hangers are fixed in position on each casing.There are however applications where a fixed position casing hanger isunsatisfactory, because the hang-off point of one casing on another mayrequire to be adjusted. Such drilling wellheads have to accommodate acasing with an undetermined hang-off point, it has been known to usecasing slip-type support mechanisms.

Wellheads are used in oil and gas drilling to suspend casing, seal theannulus between casing strings, and provide an interface with the BOP.The design of a wellhead is generally dependant upon the location of thewellhead and the characteristics of the well being drilled or produced.One specific type of wellhead is a unitized wellhead for platform orland applications.

Unitized wellheads are composed of several individual components,including a wellhead housing that is used to support a number of casinghangers and tubing hangers. The hangers support the weight of the casingand tubing, and pass loads back to the wellhead housing. Annulus sealsseal the annular spaces between casing and tubing strings.

Conventional land or platform wellheads are either slip-typeconventional wellheads or through-the-BOP multi-bowl wellheads.

Slip-type wellheads use casing slips to support casing strings. Theseslips are friction wedges that “grip” the top of a casing string and useslip teeth to bite into the casing. Wellheads of this type requirehigher-risk operations, as they require lifting the BOP to installcasing slips and annulus seals. The seals that are used with slip-typecasing hangers must be actively maintained throughout the field life ofthe well.

Multi-bowl type wellheads feature reduced-risk operations, as the BOPdoes not need to be lifted to set casing slips. Instead of using slips,a multi-bowl wellhead uses a fixed landing shoulder in the wellheadhousing to support the first casing hanger. All other casing hangers arestacked on top of this initial casing hanger. The seals installed onmulti-bowl wellheads can be more dependable than those installed inslip-type wellheads, but are still often unreliable, due toeccentricities in the casing hanger/wellhead alignment and unreliabilityin the seal setting mechanisms. As the initial load shoulder mustsupport the weight of all casing strings and any loads due to testpressures, this load shoulder must intrude into the bore of the wellheadquite a bit. This can create an operational restriction that limitsoperations through this well.

Various sealing devices are known and employed in such wellheads. Oneexample of a sealing assembly is shown and described in U.S. Pat. No.4,913,469, wherein a wellhead slip and seal assembly includes a slipassembly with slips supported within a slip bowl and a seal assemblypositioned above the slip assembly and interconnected thereto forsupporting the slip assembly, the seal assembly includes two segmentsconnected to form the seal ring and each of the segments includesarcuate elements embedded in a resilient material which forms an innerseal in an inner groove. The segments of the slip bowl include segmentsinterconnected by toe nails and the seal ring includes pin and recessconnection for connecting the two segments together.

It is also known from European Patent No. 0 251 595 to use an adjustablelanding ring on a surface casing hanger to accommodate a space-outrequirement when the casing is also landed in a surface wellhead.

More recently, and as shown and described in my U.S. Pat. Nos. 6,092,596and 6,662,868, an external clamp for clamping two concentric tubes,typically two concentric tubes in an oil or gas well, has two axiallymovable tapered components which can be pulled over one another in anaxial direction to provide a contraction of internal diameter whichgrips the smaller diameter tube.

Another example of a sealing system is shown and described in U.S. Pat.No. 5,031,695, wherein a well casing hanger with a wide temperaturerange seal element is energized by axial compression with apre-determined initial portion of the casing hang load, the remainingportion of that hang load then being transferred to the wellhead orother surrounding well element without imposition on the seal element.

U.S. Pat. No. 6,488,084 shows and describes a casing hanger adapted forlanding on a load shoulder in a wellhead to seal and support a string ofcasing. The casing hanger has a lower ring for landing on the loadshoulder, the lower ring having an upward facing surface. A plurality ofcircumferentially spaced recesses are in the upward facing surface ofthe lower ring, each of the recesses having a base. A seal is located onthe lower ring and has a plurality of holes that register with therecesses in the upward facing surface of the lower ring. A slip assemblybowl has a wedging surface that carries a plurality of slip members. Theslip members grip the casing and cause the bowl to transmit downwardforces from the casing to the seal to axially compress and energize theseal. Fasteners extend from the lower ring through apertures provided inthe seal into threaded apertures provided in a downward facing surfaceof the bowl to secure the lower ring to the slip assembly but allowrelative axial movement between the bowl and the lower ring. A pluralityof substantially cylindrical stop members are located in the holes inthe seal and in the recesses of the lower ring. The stop members aresecured into threaded holes formed in the shoulder ring and contact thebases of the recesses to limit the compression of the seal to apredetermined amount.

SUMMARY OF THE INVENTION

The subject invention is directed to a method and apparatus for a sealassembly for a unitized wellhead system for land or platformapplications utilizing a friction grip technology to create maintainablemetal-to-metal seals with finely-controlled contact stresses, lock-downcasing and tubing hangers, support test loads to minimize the size oflanding shoulders required, and to rotationally lock casing hangers toprovide simplified running procedures.

The subject invention that combines the benefits of a slip-type wellheadand a multi-bowl type wellhead and is able to provide numerousadvantages by using radial compression of the wellhead to create sealsand support load.

In its simplest form, the invention provides the apparatus and methodfor accomplishing a circumferential seal between two substantiallyconcentric members by externally activating the seal once the twomembers are in position. In a typical configuration, a wellhead housingaccommodates and supports a concentric tubing hanger. The tubing hangermay be supported within the wellhead in any of the conventional ways.

One suitable method for supporting the tubing hanger in the well is theclamping mechanism shown and described in my previously mentioned U.S.Pat. Nos. 6,092,596 and 6,662,868, incorporated herein by reference.Using the system there described, a friction fit is provided between theinner diameter of the wellhead housing and the outer diameter of thetubing hanger. Once properly positioned, a compressor system mounted onthe exterior of the wellhead housing is activated, whereby the a cam orramp surface on the compressor system is moved axially relative to amated cam surface on outer circumference of the wellhead housing tocompress the wellhead housing radially inward for engaging and clampingthe tubing hanger along coextensive surfaces.

The present invention is directed to a sealing mechanism comprising acompression system such as that shown in my aforementioned patents,metal-to-metal sealing members, and where desired, redundant resilientseals. In the preferred embodiment the sealing members are integral,machined surface on the outer circumferential wall of the tubing hangerand inner circumferential wall of the wellhead housing. The sealingsurface extends circumferentially about the walls. The sealing surfaceof the tubing hanger is best designed to clear the inner diameter of thewellhead housing, i.e., there is not any radial interference between thesealing surface of the tubing hanger and the interior wall of thewellhead housing. This preserves the integrity of the seal duringassembly. Once the tubing hanger is positioned in the wellhead housing,the seal is activated by the compressor system, compressing the wellheadhousing radially inward to engage the seal.

The sealing assembly of the subject invention provides for a flexibledesign that can be used for a variety of specific applications, as willbe described herein. The simple design promotes dependability andreduces size of the overall architecture of the well. The resultingwellhead assembly has near-zero eccentricity between hangers and housingwith near-zero torque and minimal axial setting load required toenergize metal-to-metal annular seals

The sealing assembly may include external test capability formetal-to-metal annular seals.

It is an important aspect of the invention that the sealing mechanism isactivated by external lockdown and sealing activation. The rigidlockdown eliminates annular seal fretting, with contact stress evenlydistributed around seal perimeter.

The sealing assembly permits controlled and monitored application ofseal loading.

The annular seals are maintainable throughout field life.

A minimal number of running tools are required since hangers are lockedin place torsionally. A high-torque connection, e.g., a standard casingcoupling on the end of a standard casing string, can be used to run thehangers.

It is an important feature of the design that the primary load shouldercan be smaller than conventional multi-bowl load shoulders, as much ofthe load is supported through the various friction-grip interfaces. Thissmaller load shoulder means that the bore through the wellhead isincreased, allowing the first casing string run through the wellhead tobe larger in size. Alternately, a smaller load shoulder can allow theouter diameter of the wellhead to be decreased while maintaining thediameter of the casing, resulting in a smaller overall size.

The friction and gripping areas function over a length. Therefore, ifthe first casing hanger is landed high, subsequent casing/tubing hangerscan tolerate this stack-up error by landing and sealing at slightlydifferent places along the functional bore length.

The tubing hanger can be nested to reduce the work-over stack dimension.

The friction grip area supports test loads on the tubing hangerpermitting the tubing hanger load shoulder to be smaller than it priorart configurations. More space is then available in the tubing hanger tomaximize the number of control line penetrations through the tubinghanger.

The design of the subject inventions minimizes the number of wellheadpenetrations. All contingency procedures can be performed through theblow out preventers (BOP's).

Due to minimizing stress and torque, the system is a fatigue resistantdesign for dynamic applications. The flexible design allowsincorporation of tensioned casing and tubing hangers.

In the preferred compression system, the use of hydraulic pistons andlock nuts to activate and lock the flanges allows for a simplifiedflange design.

The push-through wear bushing does not need to be retrieved, saving anoperation.

Internal tubing hanger lockdown can be accomplished without a dedicatedhandling tool and without potential control line damage

Improved safety, with tubing back-side test, is achieved without the useof a temporary seal or temporary lockdown mechanism on tubing hanger.

Other features of the invention will be readily apparent from theaccompanying drawings and detailed description of the preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-section of a wellhead showing the sealsystem in detail.

FIG. 2 is a cross-section of a typical wellhead configurationincorporating the seal system of the subject invention.

FIG. 3 is an enlarged fragmentary view of the seal system of FIG. 1, andcorresponds generally to FIG. 1.

FIG. 4 is a cross-section of a typical wellhead configurationincorporating the seal system of the subject invention with the tubinghanger nested to reduce the work-over stack dimension.

FIG. 5 is a cross-section of the wellhead of FIG. 4 taken at a 90 degreerotation from that of FIG. 4.

DESCRIPTION OF THE INVENTION

A simplified, diagrammatic view of the seal system to the subjectinvention is shown in FIG. 1. In its simplest form, the inventionprovides the apparatus and method for accomplishing a circumferentialseal between two substantially concentric members by externallyactivating the seal once the two members are in position.

With specific reference to FIG. 1, a wellhead 1 includes having anexternal sealing apparatus 10 for clamping a tubular casing 4 of a firstdiameter within a tubular casing (here the wellhead 1) of largerinternal diameter. The outer tubular member has an inner circumferentialwall with a sealing zone 83. The inner tubular member is adapted to bepositioned substantially concentrically within the outer tubular memberhaving an outer circumferential wall with a sealing zone 28. Thecircumferential compression system 10 is mounted outwardly of the outertubing member and operable to be activated for compressing the outertubular member into contact with the inner tubular member for engagingthe sealing zones therein and activating a seal between the outertubular member and the inner tubular member. The sealing zone on eachtubular member may be a metal sealing surface on each of said tubularmembers for defining a metal-to-metal seal when the compressions systemis activated. Where desired, the wellhead sealing system may include oneor more resilient seal members 84, 85 in the sealing zone of one of thetubular members and extending outwardly therefrom toward the othertubular member, wherein the resilient seal member is adapted to becompressed between the two tubular members when the compression systemis activated. Where multiple resilient sealing members are used, a gap91 is created between the resilient seal members when the compressionsystem is activated. A test port 114 may be provided for communicatingthe gap with the exterior of the assembly for testing the integrity ofthe seal when activated. In the preferred embodiment the compressionsystem comprises a wedge surface 15 and a flange 14 adapted for engagingthe wedge, one of said wedge and flange being each located on one of theouter tubular member and the compression system, whereby the tubularmember is compressed radially inwardly upon relative axial movementbetween the wedge and the flange. The preferred method for activatingthe compression system is a hydraulic ram adapted for causing axialmovement between the wedge and the flange. The system includes apositive lock 21 for locking the wedge and flange in position once theseal has been engaged.

In its broadest sense the invention is a method for providing anexternal sealing device for concentric tubular members in a wellhead.The method comprises placing sealing zones on the mated surfaces of aplurality of concentric tubular members in radial alignment with oneanother and compressing the outermost tubular member toward the centralaxis of the concentric tubular members for engaging the sealing zoneswith one another. As described above, in the preferred embodiment themethod includes the step of locking the compressed assembly in sealingposition. Where desirable, a redundant resilient seal is positioned inthe sealing zone. When a plurality of axially spaced resilient seals arelocated in the sealing zone, the gap between the resilient seals may beported to the exterior of the system.

As shown in FIG. 1, and by way of example, a wellhead housing 1accommodates and supports a concentric tubing hanger 4. As will befurther described, additional concentric tubular members may also besealed using the system of the subject invention. The tubing hanger maybe supported within the wellhead in any of the conventional ways. Onesuitable method for supporting the tubing hanger in the well is theclamping mechanism shown and described in my earlier U.S. Pat. No.6,092,596, incorporated herein by reference. Using the system thereindescribed, a friction fit is provided between the inner circumferentialwall 83 of the wellhead housing and the outer circumferential wall 28 ofthe tubing hanger 4. Once properly positioned, the compressor system 10mounted on the exterior of the wellhead housing 1 is activated by thethreaded driver 20, 21, whereby the compression flange 14 on thecompressor system is moved axially relative to the compression wedge 15on outer circumference of the wellhead housing to compress the wellheadhousing radially inward for engaging and clamping the tubing hangeralong the coextensive surfaces 28 and 83. As shown in my aforementionedpatents, the compression system may comprise an annular, axiallytapering surface, an axially movable sleeve surrounding the outer wallof the wellhead and has a corresponding tapering surface facing theouter wall, and a driver for producing relative axial movement betweenthe tapering surfaces to exert a radial compressive force to the outerwall of the wellhead. The means for producing relative axial movementcomprises a pressure chamber between the sleeve and the wellhead, andmeans for pressurising the chamber with hydraulic pressure.Alternatively, the means for producing relative axial movement maycomprise a flange on the sleeve, a flange on the wellhead, and means forapplying a mechanical force between the flanges to move the sleeveaxially along the wellhead.

The present invention is directed to the sealing mechanism comprisingthe compression system 10, the metal-to-metal sealing member 29, andwhere desired, redundant resilient seals 84 and 85. In the preferredembodiment the sealing member 29 may is an integral, machined surface onthe outer wall 28 of the tubing hanger. The sealing surface extendscircumferentially about the outer wall of the tubing hanger. The sealingsurface is best designed to clear the inner wall of 83 of the wellheadhousing, i.e., there is not any radial interference between the sealingsurface of the tubing hanger and the interior wall of the wellheadhousing. This preserves the integrity of the seal during assembly. Oncethe tubing hanger 4 is positioned in the wellhead housing 1, the seal isactivated by driving the compression flange 14 of the compressor system10 relative to the compression wedge 15 mounted on the wellhead housing1, forcing the wellhead housing to compress radially inward about theentire circumference and engage the seal.

In the preferred embodiment, the metal-to-metal seal includes mated andcomplementary sealing surfaces 29 and 90 on both the exterior wall ofthe tubing hanger and the interior wall of the wellhead housing.

Resilient back up seals 84, 85 may also be provided. As shown in FIG. 1,the exterior wall of the tubing hanger includes channels 86, 87, forreceiving an the resilient o-ring type resilient seal 84, 85. Thechannels and O-rings could also alternatively be housed in the interiorwall of the wellhead housing. The resilient seal system is alsoactivated by the compressor system 10.

It is also desirable to provide a seal test port 114 in communicationwith the seal for testing its integrity once activated.

The seals are released by decompressing the compressor system 10 towithdraw the ramp surface 14 axially downward from the ramp surface 16via the screw drive system 21. The drive means may be any of a number ofsystems which support the exertion of circumferential pressure on theouter wall of the wellhead. Examples of such systems are shown anddescribed in my U.S. Pat. No. 6,662,868 and copending application U.S.Ser. No. 10/721,443. All of these are incorporated by reference herein.

It is, therefore, the essence of the invention to provide a sealingmechanism for sealing the annulus between two relatively concentrictubular members by activating and engaging a sealing member via anexternal force applied to the assembly for compressing the outer memberinto the inner member.

It should be noted that the seal mechanism must be distinguished fromthe clamping mechanism described in the aforementioned patents. As willbe readily understood, sufficient clamping can be accomplished bycompressing the outer member into the inner member whether or not fullcircumferential contact is achieved. It is the important enhancement ofthe subject invention that means are provided to assure complete contactalong the circumferential walls of the two member to effect a seal oncethe compression is completed.

FIG. 2 depicts a simple configuration of a three-string wellhead systemutilizing the clamping system of my aforementioned patents and thesealing system of the present invention. The main components of thissystem are a wellhead housing 1, a production casing hanger 2 withannulus seal assembly 3, and a tubing hanger 4. The entire assembly issupported on a base plate 5 that sits on the conductor string 6.

A load shoulder 37 on the support plate supports the wellhead housing.The wellhead housing 1 supports the weight of the intermediate casingstring 7 in a traditional manner (in this case, via a threaded casingcoupling connection in the bottom of the wellhead housing). The exteriorof the wellhead housing features two sets of annulus access ports 8 and9, two clamping compression systems 10 and 11, a control-line accessport 12, two sets of external seal test ports 113 and 114, and athread-on flange profile up. A thread on flange 35 attaches to thisprofile to interface with the tree adapter 33.

The bore of the wellhead housing is featured with a number of sealingprofiles and lockdown profiles for the casing hanger, seal assembly, andtubing hanger. These bores may be on a series of steps so that eachhigher bore is on a slightly larger diameter, therefore protected fromoperations on the smaller diameter bores. At the top of the wellheadhousing bore is an index shoulder 22 for the tubing hanger neck seal anda gasket sealing profile. At the bottom of the wellhead housing bore isa load shoulder 23 that is sized to support the casing weight of theproduction casing string only. Any additional axial load (for instanceload from other casing strings or from test pressures) passes throughthe friction-grip lockdown areas.

The production casing hanger 2 features a casing thread profile down forsupport of the production casing string 24 and a casing thread profileup to interface with the casing hanger's casing running string (notshown). The exterior of the casing hanger features a load shoulder thatis slotted to allow flow-by and cement returns to pass the exterior ofthe casing hanger as it is being run. The external surface of the loadshoulder area 25 is a controlled surface featuring a friction profile.When the casing hanger is landed, this friction surface is parallel to amating surface in the bore of the wellhead housing. External compressionof the wellhead housing provided by the lower compression cartridge 111forces the two surfaces to be perfectly concentric and brings them intocontact. Friction at this interface provides rotational and axiallock-down support for the casing hanger, as well as additional loadsupport for production casing weight and test loads on the productioncasing hanger. Above the casing hanger load shoulder is a profile forthe annulus seal system 3.

The annulus seal 3 fits between the production casing hanger 2 and theinner bore of the wellhead housing 1. The seal features two sets of sealprofiles 115, 116 on both the inner and outer diameters, respectively.The outer diameter and inner diameter seal profiles feature two pairseach of metal-to-metal seals as well as resilient seal back-ups 118,119. A port 113 between the two sets of seals allows external testing ofall seals created by the seal assembly. These seal profiles do not haveinitial radial interference with either the casing hanger or thewellhead housing. Rather, interference (and radial contact pressure) isprovided by external compression of the wellhead housing through the useof the lower compression cartridge 11. An extended neck 120 on the sealassembly protrudes above the top of the casing hanger. This extendedneck features ports 122 to allow communication between theproduction/tubing annulus and the upper annulus access port 8 in thewellhead housing. The top of the seal assembly serves as a landingshoulder 124 for the tubing hanger 4 at load shoulder 26.

The tubing hanger 4 supports the tubing string 27 with a threadedconnection down. The thicker main body 125 of the tubing hanger providesa load shoulder 26 that lands on top of the production casing hangerannulus seal assembly on landing shoulder 124. This load shouldersupports full tubing string weight only. Any additional axial loads (forinstance, loads due to test pressure) are supported by the friction-griplockdown area. The outer diameter of the thick section 125 of the tubinghanger features a friction-lock profile 28 below a sealing profile 29.The friction profile is a machined surface suitable for support offriction loads. The sealing profile consists of a pair of metal-to-metalseal bumps with resilient back-ups, as described with above and shownmore clearly in FIGS. 1 and 3. Both of these profiles are parallel tomating surfaces on the wellhead housing bore, and have no initialinterference. When the upper compression cartridge 10 is activated, thatsection of the wellhead housing is compressed inwards to contact thetubing hanger. Contact pressure along this interface forces the piecesto be concentric, provides axial and rotational lockdown of the tubinghanger, and activates the metal-to-metal seals with resilient back-ups.The friction interface supports any test pressure loads on the tubinghanger.

Hydraulic control lines 30 pass through the tubing hanger body in aconventional manner. The tubing hanger features an extended neck 126upwards. This neck features a tubing connection box up to interface withthe tubing running string (not shown). Below this threaded box is a sealprofile to accept the tubing hanger neck seal.

The tubing hanger neck seal 31 sits on a support ring 32 that is carriedon the tubing hanger neck and indexes on a load shoulder in the wellheadhousing bore. The seal sits on the upper face of this support ring, andfeatures metal-to-metal seal profiles on both the straight innerdiameter and the tapered outer diameter. A port 127 between these sealprofiles allows external testing of all seals created by the tubinghanger neck seal via an external test port 36 in the Christmas treeadapter 33. This seal is activated as the Christmas tree adapter 33 isdrawn by studs and nuts 34 down onto the wellhead housing. Movement overthe tapered external surface of the tubing hanger neck seal compressesthe seal inwards and creates high radial contact pressures on both theseal inner diameter and the seal outer diameter.

FIG. 3 is an enlarged a detail of the system shown in FIG. 2, generallyin the area of the upper compressor system 10. FIG. 3 is generally ofthe same cross-section of FIG. 1, but with all of the detail of thewellhead housing of FIG. 2.

Each POS-GRIP compression system is composed of a compression flange 14and a compression wedge 15. The compression flanges are rings withtapered inner surfaces that mate with the tapered outer surfaces of thecompression wedges. Axial movement of the compression flanges over thecompression wedges compresses the compression wedges inwards, in turncompressing a portion of the wellhead housing 1 inwards (within thewellhead housing's elastic range). The compression systems may beconfigured with a split spacer ring 16 between the compression wedge andthe wellhead housing, as shown in the top compression system 10 of FIG.2. The split spacer rings have minimal hoop stiffness, and simply passthe radial contact loads from the compression wedge into the wellheadhousing.

The compression flanges have handling profiles 17 on the flange outerdiameters. These handling profiles interface with a release tool (notshown) that can be used to push the flanges apart, releasing thecompression. The compression flanges also have activation and lockingprofiles 18 cut into the wide end of the flanges. These profiles accepta set of small hydraulic pistons (not shown) during activation. Thesehydraulic pistons react against the thick section of the wellheadhousing in the region of the upper annulus access port 8, see FIG. 2.When pressure is applied to a set of hydraulic pistons, the associatedcompression flange is pushed away from the thick section of the wellheadhousing into the “activated” position. Once the compression flange hasbeen moved into its activated position, mechanical lock nuts 19 replacethe hydraulic pistons in the locking profiles, and are used to lock theflange in the activated position.

The lock nuts consist of a male thread member 20 and a female threadmember 21. The male thread member has a threaded length and a flat faceat one end to sit on the wellhead housing. The female thread member hasthreads to mate with the male thread member and a flat face to react onthe compression flange. Rotation of the female thread member on the malethread member allows the lock nut to adjust in length, to fill whatevergap is developed between the wellhead housing and the compressionflanges during activation of the compression system. Once the lock nuthas been adjusted to the necessary length, it effectively locks thecompression flange in its current position, so that the hydraulicpistons may be removed.

FIGS. 4 and 5 depict two separate sections of a more involvedconfiguration of a four-string wellhead. The main components of thissystem are a wellhead housing 38, a push-through wear bushing 39, anintermediate casing hanger 40 with annulus seal assembly 41. The annulusseal assembly is of the same configuration as that shown in FIG. 2 andis activated in a similar manner by the lower compression system 11.There is also a production casing hanger 42, a seal and support sub 43,and a tubing hanger 44.

The assembly shown in FIGS. 4 and 5 uses an alternate means of wellheadsupport. In this case, the entire assembly is supported on a frictionsupport mechanism 45 that connects the bottom of the wellhead housing tothe top of a large-diameter casing string 46. The friction supportmechanism consists of a gripping sub 47, a compression sub 49, and a setof studs and nuts 50. This gripping system comprising gripping sub 47,compression sub 49 and the driver 50, operates in accordance with thegripping system shown and described in my aforementioned patents. Thegripping sub is connected to the inner diameter of the wellhead housing38 via a threaded profile at 130 with a metal-to-metal seal. The lowerportion 131 of the gripping sub consists of a friction and sealingprofile on the inner diameter and a tapered surface on the outerdiameter. The friction profile diameter fits as a socket around thecasing string 46. The tapered diameter mates with a tapered surface onthe compression sub 49. As the compression sub moves upwards over thetaper, the gripping sub is compressed inwards. This closes the gapbetween the gripping sub and the outer diameter of the casing, andcreates a high radial contact pressure between the two pieces. This highradial contact pressure provides a metal-to-metal seal between thegripping sub and the casing. Friction at this interface locks the piecestogether axially and rotationally.

A set of studs and nuts 50 connect the compression sub 49 to thewellhead housing 38. It is movement of the nuts along the studs thatcauses the compression sub to move upwards along the tapered compressionsub/gripping sub interface.

The wellhead housing 38 is largely the same as that shown in FIG. 2. Thewellhead housing in FIGS. 4 and 5 features a third annulus access port52 (FIG. 4) to allow access to the additional annulus created in thefour-string configuration. This annulus access port is located at 90degrees from the production casing/intermediate casing annulus accessport 51 (FIG. 5). Both ports may be located at the same height as shownin these drawings. There is also one additional test port 52 (FIG. 4)through the wellhead housing to test an additional set of seals 135 onthe tubing hanger.

This wellhead housing also demonstrates a different means of providing areaction point for the hydraulic activation pistons and mechanical locknuts. Instead of having a very thick section integral to the wellheadhousing (as was shown in FIG. 2), this wellhead housing features aseries of split flange sections 54 that fit in a dovetail groove 55 in aslightly thicker portion 136 of the wellhead housing. These flanges maythen be bolted into place. At locations where annulus access port passesthrough the wellhead housing, a flat is machined to allow an annulusaccess valve to be bolted in place.

This system is used with a push-through wear bushing. This wear bushingprotects the wellhead bore when drilling for the intermediate casingstring. The wear bushing 39 is simply a thin sleeve with a thick topsection. The bottom of the thin sleeve passes through the wellheadhousing minimum inner diameter. A set of resilient seals 57 at the topof the wear bushing 39 prevents fluids from entering the protected area.The wear bushing may be supported in one of two ways. First, a pinthrough one of the annulus access ports can latch into a profile on theouter diameter of the wear bushing. This pin can then be removed whenthe wear bushing is ready to be moved out of the way. Alternately, thethick upper portion of the wear bushing may be gripped by thecompression system 11. This system is released when the wear bushing isready to be moved out of the way.

The thicker portion at the top of the wear bushing serves as a loadshoulder 138 for the intermediate casing hanger. The wear bushing isreleased when the intermediate casing hanger is run. The load shoulder140 on the intermediate casing hanger lands on the top of the matingload shoulder on the wear bushing and pushes the wear bushing downwardsuntil the thick portion of the wear bushing is sandwiched between thelower load shoulder 142 on the wellhead housing and the load shoulder140 on the intermediate casing hanger. These shoulder thicknesses areall sized to support full intermediate casing weight only. Anyadditional load on the intermediate casing hanger (due to loads fromadditional casing strings and seal test loads) is supported by thefriction interface which is activated by the compression system 11.

The intermediate casing hanger 150 and intermediate casing hanger sealassembly 41 are largely identical to the production casing hanger 2 andproduction casing hanger annulus seal assembly 3 as discussed in FIG. 2.The intermediate casing hanger features a profile 58 on the innerdiameter to land the production casing hanger 42. As a hanger does notland on top of the annulus seal as one did in the configuration of FIG.2, the annulus seal is shorter, and does not have the requirement ofports for annulus access.

The production casing hanger 42 features a casing thread profile downfor support of the production casing string 59. At the top end of theproduction casing hanger, there is a casing coupling box 152 tointerface with the seal and support sub 43 and an external runningthread profile to interface with the casing hanger's running tool (notshown). The exterior of the production casing hanger features slots toallow flow-by and cement returns to pass as the hanger is being run.

Held in a profile on the exterior of the production casing hanger is asplit-ring landing mechanism 60 (FIG. 5). This outwardly biased splitring is held inwards by the casing hanger running tool while the hangeris being run. This allows the production casing hanger to passcompletely through the bore of the intermediate casing hanger, and thenbe pulled back to the mating landing profile, thus applying tension tothe production casing string. When the production casing hanger isproperly located in the bore of the intermediate casing hanger, theoutwardly-biased split ring is disengaged from the running tool. Thesplit ring springs outwards and engages the mating profile in the boreof the intermediate casing hanger. This split ring supports intermediatecasing string weight only. Any additional loads on the intermediatecasing hanger (for instance, loads due to the tubing string or any sealtest loads) are carried by the seal and support sub.

The seal and support sub 43 has a casing coupling pin down. Thisthreaded and sealing connection is made up to the mating box 152 in thetop of the production casing hanger 150. On the inner diameter abovethis coupling is a running profile 61 to mate with a running tool (notshown). Above this running profile, ports 62 (FIG. 4) pass from the sealand support sub inner diameter to the outer diameter to allowcommunication between the production casing/tubing annulus and theannulus access port 156.

At the outer diameter of the seal and support sub, these ports passbetween a pair of metal-to-metal seals at seal assembly 160. The outerdiameter of the seal and support sub features four sets ofmetal-to-metal seals 162 with resilient backup 63. The annulus accessports pass between the middle set of seals. The set of seals on eitherside of the annulus access port straddle external test ports in thewellhead housing wall, enabling testing of all sets of seals. Below allof these sealing profiles is a friction profile 64, consisting of amachined surface suitable for support of friction loads.

Both of these profiles are parallel to mating surfaces on the wellheadhousing bore, and have no initial interference. When the uppercompression cartridge 165 is activated, that section of the wellheadhousing is compressed inwards to contact the seal and support sub.Contact pressure along this interface forces the pieces to beconcentric, provides axial and rotational lockdown of the seal andsupport sub, and activates the metal-to-metal seals with resilientback-ups. The friction interface supports any test pressure loads on theseal and support sub and any weight from the tubing hanger.

The inner diameter of the support sub is a bowl that serves as a landingshoulder 170 for the tubing hanger 65. Above this landing shoulder is abore with both a friction grip profile 66 and a sealing profile 67 forthe tubing hanger.

The tubing hanger 65 is very similar to the tubing hanger 4 shown inFIG. 2. The tubing hanger 65 has a reduced outer diameter, allowing itto be run through a smaller blow out preventer (BOP). This smallertubing hanger is landed, locked down, and sealed inside the seal andsupport sub rather than inside the wellhead housing bore. In order tohave capability to test the metal-to-metal seals on the tubing hangerouter diameter, a port 68 in the tubing hanger passes from the top faceto intersect a test port that passes between the two sets of seals onthe tubing hanger outer diameter.

To activate the seals and friction grip inside the seal and support subrequires a two-stage operation of the upper compression system 165. Thefirst stage of activation compresses the wellhead housing inwards togrip, support, and seal the seal and support sub. During the secondstage of activation, the compression system is activated further. Thisadditional activation compresses through the seal and support sub,compressing the inner diameter of the seal and support sub inwards togrip the tubing hanger. This second-stage compression provides the forcenecessary to activate the metal-to-metal seals and the friction-gripsupport. The tubing hanger neck seal is identical to that shown FIG. 2.

From the foregoing description it will be readily understood that theplatform wellhead design of the subject invention has numerousenhancements and features providing substantial advantages over thewellhead designs of the prior art. The wellhead as described hereinachieves these advantages by moving load support and seal energizationfunctions to the exterior to the wellhead housing. This results inmaximization of useable bore space and excellent control of annular sealloading. These improvements result in the following advantages andfeatures, among others:

-   -   flexible design can be used for a variety of specific        applications.    -   Simple design promotes dependability and reduces size.    -   Zero eccentricity between hangers and housing.    -   Zero torque and minimal axial setting load required to energize        metal-to-metal annular seals.    -   External test capability for metal-to-metal annular seals.    -   External lockdown and sealing activation Rigid lockdown        eliminates annular seal fretting.    -   Contact stress evenly distributed around seal perimeter.    -   Controlled and monitored application of seal loading.    -   Annular seals maintainable throughout field life.    -   Minimal number of running tools required-since hangers are        locked in place torsionally, a high-torque connection (in this        case a standard casing coupling on the end of a standard casing        string) can be used to run the hangers.    -   The primary load shoulder can be quite a bit smaller than        conventional multi-bowl load shoulders, as much of the load is        supported through the various friction-grip interfaces. This        smaller load shoulder means that the bore through the wellhead        is increased, allowing the first casing string run through the        wellhead to be larger in size. Alternately, a smaller load        shoulder can allow the outer diameter of the wellhead to be        decreased, resulting in a smaller overall size.    -   The friction and gripping areas function over a length.        Therefore, if the first casing hanger is landed high, subsequent        casing hangers/tubing hangers can tolerate this stack-up error        by landing and sealing at slightly different places along the        bore length.    -   As shown in FIG. 4, the tubing hanger can be nested to reduce        the work-over stack dimension.    -   Due to the fact that the friction grip area supports test loads        on the tubing hanger, the tubing hanger load shoulder can be        smaller than it would normally be. This means that more space is        available in the tubing hanger to maximize the number of control        line penetrations through the tubing hanger.    -   Minimum number of wellhead penetrations.    -   Contingency procedures can all be performed through the BOP's.    -   Fatigue resistant design for dynamic applications.    -   Flexible design allows incorporation of tensioned casing and        tubing hangers (for instance as shown in FIG. 4).    -   Use of hydraulic pistons and lock nuts to activate and lock        flanges allows simple flange design.    -   Push-through wear bushing does not need to be retrieved, saving        an operation.    -   Internal tubing hanger lockdown without dedicated handling tool        and potential control line damage    -   Improved safety, with tubing back-side test achieved without use        of temporary seal or temporary lockdown mechanism on tubing        hanger.

While certain features and embodiments of the invention have beedescribed in detail herein, it should be understood that the inventionincludes all modifications and enhancements within the scope of thefollowing claims.

1. A wellhead having an external sealing apparatus for clamping a tubingmember a first diameter within a tubing member of larger internaldiameter, the arrangement comprising an outer tubing member having aninner circumferential wall with a sealing zone therein; an inner tubingmember adapted to be positioned substantially concentrically within theouter tubing member having an outer circumferential wall with a sealingzone therein; a circumferential compression system mounted outwardly ofthe outer tubing member and operable to be activated for compressing theouter tubing member into contact with the inner tubing member forengaging the sealing zones therein and activating a seal between theouter tubing member and the inner tubing member.
 2. The wellhead ofclaim 1, wherein the sealing zone is a metal sealing surface on each ofsaid tubing members for defining a metal-to-metal seal when thecompressions system is activated.
 3. The wellhead of claim 1, furtherincluding a resilient seal member in the sealing zone of one of thetubing members and extending outwardly therefrom toward the other tubingmember, wherein the resilient seal member is adapted to be compressedbetween the two tubing members when the compression system is activated.4. The wellhead of claim 3, including a second resilient seal memberaxially in the sealing zone and spaced axially from the first sealingmember, creating a gap between the resilient seal members when thecompression system is activated.
 5. The wellhead of claim 4, including atest port for communicating the gap with the exterior of the assemblyfor testing the integrity of the seal when activated.
 6. The wellheadapparatus of claim 1, wherein the compression system comprises a wedgesurface and a flange adapted for engaging the wedge, on of said wedgeand flange being each located on one of the outer tubular member and thecompression system, whereby the tubular member is compressed radiallyinwardly upon relative axial movement between the wedge and the flange.7. The wellhead of claim 6, wherein the compression system is ahydraulic ram adapted for causing axial movement between the wedge andthe flange.
 8. The wellhead of claim 7, further including a positivelock for locking the wedge and flange in position once the seal has beenengaged.
 9. The wellhead of claim 1, wherein the compression systemcomprises annular, axially tapering surface, an axially movable sleevesurrounding the outer wall of the wellhead and has a correspondingtapering surface facing the outer wall, and a driver for producingrelative axial movement between the tapering surfaces to exert a radialcompressive force to the outer wall of the wellhead.
 10. The wellhead ofclaim 9, wherein the means for producing relative axial movementcomprises a pressure chamber between the sleeve and the wellhead, andmeans for pressurising the chamber with hydraulic pressure.
 11. Thewellhead of claim 10, wherein the means for producing relative axialmovement comprises a flange on the sleeve, a flange on the wellhead, andmeans for applying a mechanical force between the flanges to move thesleeve axially along the wellhead.
 12. The wellhead of claim 11, whereina locknut is provided to lock the relative positions of the sleeve andthe wellhead, once these components have been brought into an active,seal engaging position.
 13. A method for sealing concentric tubularmembers in a wellhead, comprising the steps of: a. placing sealing zoneson the mated surfaces of a plurality of concentric tubing members inradial alignment with one another; b. compressing the outermost tubularmember toward the central axis of the concentric tubing members forengaging the sealing zones with one another.
 14. The method of claim 9,including the step of locking the compressed assembly in sealingposition.
 15. The method of claim 9, including the step of placing aredundant resilient seal in the sealing zone.
 16. The method of claim 9,including the step of placing a plurality of axially spaced redundantseals in the sealing zone.
 17. The method of claim 12, including thestep of porting the space between the axially spaced redundant seals tothe exterior of the assembly.
 18. A wellhead assembly comprising a wellhead, a first tubing member supported in the wellhead and at least oneadditional tubing member concentric with and of a smaller diameter thanthe first tubular member, the assembly comprising: an external sealingapparatus for clamping a tubing member of a first diameter within atubing member of larger internal diameter, the arrangement comprising anouter tubing member having an inner circumferential wall with a sealingzone therein; an inner tubing member adapted to be positionedsubstantially concentrically within the outer tubing member having anouter circumferential wall with a sealing zone therein; a clamp assemblyfor securing a rigid, generally cylindrical outer tubing member to arigid, generally cylindrical inner tubing member, the clamp assemblycomprising: an annular compression member radially surrounding the outertubing member, the annular compression member having a pair of opposed,outwardly-facing ramp surfaces; a pair of collars radially surroundingthe compression member, each of the collars having an inwardly-facingramp surface for contacting and adjoining one ramp surface of thecompression member; means for axially moving the pair of collars withrespect to one another to cause the annular compression member to bedeformed radially inwardly to cause the outer tubing member to grip theinner tubing member in a primary clamping grip; and a locking system toprevent substantial movement of the inner tubing member with respect tothe outer tubing member.
 19. The assembly of claim 18, further includinga circumferential compression system mounted outwardly of the outertubing member and operable to be activated for compressing the outertubing member into contact with the inner tubing member for engaging thesealing zones therein and activating a seal between the outer tubingmember and the inner tubing member.
 20. The assembly of claim 18,wherein the clamp assembly is also operable to be activated forcompressing the outer tubing member into contact with the inner tubingmember for engaging the sealing zones therein and activating a sealbetween the outer tubing member and the inner tubing member.
 21. Theassembly of claim 18 wherein the locking system comprises: an outwardlyand downwardly facing first tapered shoulder associated with the innermember; and a complementary inwardly and upwardly facing second taperedshoulder associated with the outer member, the first and secondshoulders engaging one another upon failure of the primary clampinggrip.
 22. The clamp assembly of claim 21 wherein the first taperedshoulder is formed upon an annular member that is affixed by at leastone threaded connection to the inner tubing.
 23. The clamp assembly ofclaim 22 wherein the second tapered shoulder is formed within an innersurface of the outer member.
 24. The clamp assembly of claim 18, whereinthe locking system comprises a slip member disposed between the innertubing member and outer tubing, said slip member shaped and sized tocause a mechanical gripping between the inner tubing member and outertubing in the event the primary clamping grip fails.
 25. The clampassembly of claim 24, wherein the slip member is an annular member. 26.The clamp assembly of claim 24, wherein the slip member is a segment ofan annular member.
 27. The clamp assembly of claim 24, wherein the slipmember presents a notched inner surface for gripping the inner tubingmember.
 28. The clamp assembly of claim 24, wherein the slip memberpresents a downwardly-tapered outer surface for engagement of acomplementary surface on the outer tubing.
 29. A wellhead assemblycomprising: an inner, generally cylindrical tubing; an outer, generallycylindrical tubing that radially surrounds the inner tubing member; anexternal sealing apparatus for sealing the annulus between the innertubing and the outer tubing, comprising a sealing zone on the internalwall of the outer tubing; a complementary sealing zone on the outer wallof the inner tubing a plurality of clamping arrangements secured to theouter tubing, each of said clamping arrangement being selectivelymoveable between a first position wherein the clamping arrangement doesnot secure the outer tubing to the inner tubing member and a secondposition wherein the clamping arrangement radially compress a portion ofthe outer tubing radially inwardly, each of the clamping arrangements,one of said clamping arrangements radially aligned with thecomplementary sealing zones, the clamping arrangements comprising: anannular compression member radially surrounding the outer tubing, theannular compression member having a pair of opposed, outwardly-facingramp surfaces; pair of collars radially surrounding the compressionmember each of the collars having an inwardly-facing ramp surface forcontacting an adjoining one ramp surface of the compression member; andmeans for axially moving the pair of collars with respect to one anotherto cause the annular compression member to be moved radially inwardly tocause the outer tubing to be compressed radially inwardly.
 30. Theassembly of claim 29, wherein the clamping arrangements are located atintervals along the outer tubing.
 31. The assembly of claim 29, whereinthe sealing zones are each a metal sealing surface on each of saidtubing members for defining a metal-to-metal seal when the compressionssystem is activated.
 32. The assembly of claim 29, further including aresilient seal member in the sealing zone of one of the tubing membersand extending outwardly therefrom toward the other tubing member,wherein the resilient seal member is adapted to be compressed betweenthe two tubing members when the compression system is activated.
 33. Theassembly of claim 32, including a second resilient seal member axiallyin the sealing zone and spaced axially from the first sealing member,creating a gap between the resilient seal members when the compressionsystem is activated.
 34. The assembly of claim 33, including a test portfor communicating the gap with the exterior of the assembly for testingthe integrity of the seal when activated.
 35. The assembly of claim 29,wherein the compression system comprises a wedge surface and a flangeadapted for engaging the wedge, on of said wedge and flange being eachlocated on one of the outer tubular member and the compression system,whereby the tubular member is compressed radially inwardly upon relativeaxial movement between the wedge and the flange.
 36. The assembly ofclaim 35, wherein the compression system is a hydraulic ram adapted forcausing axial movement between the wedge and the flange.
 37. Theassembly of claim 36, further including a positive lock for locking thewedge and flange in position once the seal has been engaged.
 38. Theassembly of claim 29, wherein the compression system comprises annular,axially tapering surface, an axially movable sleeve surrounding theouter wall of the wellhead and has a corresponding tapering surfacefacing the outer wall, and a driver for producing relative axialmovement between the tapering surfaces to exert a radial compressiveforce to the outer wall of the wellhead.
 39. The assembly of claim 38,wherein the means for producing relative axial movement comprises apressure chamber between the sleeve and the wellhead, and means forpressurising the chamber with hydraulic pressure.
 40. The assembly ofclaim 39, wherein the means for producing relative axial movementcomprises a flange on the sleeve, a flange on the wellhead, and meansfor applying a mechanical force between the flanges to move the sleeveaxially along the wellhead.
 41. The assembly of claim 40, wherein alocknut is provided to lock the relative positions of the sleeve and thewellhead, once these components have been brought into an active, sealengaging position.